Over‐wintering stage polymorphism of a bruchine beetle: geographical variation in optimal diapause strategy

1. Most temperate insects undergo diapause at a specific developmental stage to overcome severe winters. The bruchine beetle Bruchidius dorsalis in a warmer region in Japan, however, has diverse over‐wintering stages – non‐diapausing larvae, diapausing larvae, and diapausing adults, whereas in a cooler region, it over‐winters only as the final instar larva or adult in diapause. 2. The geographical pattern of over‐wintering stages in 12 populations was investigated over a wide range along the mainland of Japan. It revealed that a distinct geographical pattern of over‐wintering stages exists in relation to temperature conditions. In regions with warmer climates, the proportion of non‐diapausing larvae increased and B. dorsalis had a more complex over‐wintering stage structure. 3. Life cycles were also compared between two areas of Japan by field experiments. In the cooler area, the first generation over‐wintered in the diapausing larval or adult stage. Conversely, in the warmer area, diapause was induced later and some of the first‐generation adults produced second‐generation offspring before over‐wintering. 4. Based on the geographical cline of climates and the differences in cold hardiness among stages, we can demonstrate that the over‐wintering stage variation among and within populations results from an adaptive timing of diapause induction in each region, because the late larval or adult diapauses protect pupae or eggs – which unlike other stages are not cold hardy – from being produced late in the season.